The present invention relates to substrate processing. Specifically, the present invention relates to an apparatus and method for cleaning a processing chamber in a substrate processing system which reduces the time required to complete a dry-clean technique by increasing the flow-rate of cleaning gases therein.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a layer, such as a metal silicide layer like tungsten silicide (WSix), on a substrate or wafer. As is well known, such a layer can be deposited by chemical vapor deposition (CVD). In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to form the desired film. In a conventional plasma-enhanced CVD (PECVD) process, a controlled plasma is formed using radio frequency (RF) energy or microwave energy to decompose and/or energize reactive species in reactant gases to produce the desired film.
One problem that arises during such CVD processes is that unwanted deposition occurs in the processing chamber and leads to potentially high maintenance costs. With CVD of a desired film on a wafer, undesired film deposition can occur on any hot surface including the heater or process kit parts of the apparatus, because the reactive gases can diffuse everywhere in the processing chamber, even within cracks and around corners. During subsequent wafer depositions, this excess growth on the heater and/or other parts of the apparatus will accelerate until a continuous metal silicide film is grown on the heater and/or these other parts. Over time, failure to clean the residue from the CVD apparatus often results in degraded, unreliable processes and defective wafers. When excess deposition starts to interfere with the CVD system""s performance, the heater and other process kit parts (such as the shadow ring and gas distribution faceplate) can be removed and replaced to remove unwanted accumulations in the CVD system. Depending on which and how many parts need replacing and the frequency of the replacement, the cost of maintaining the substrate processing system can become very high.
In these CVD processes, a reactive plasma cleaning is regularly performed to remove the unwanted deposition material from the processing chamber walls, heater, and other process kit parts of the processing chamber. Commonly performed between deposition steps for every wafer or every n wafers, a reactive plasma cleaning procedure that is performed as a standard processing chamber operation where the etching gas is used to remove or etch the unwanted deposited material. One reactive plasma cleaning procedure is performed in situ in the processing chamber promotes excitation and/or disassociation of the reactant gases by the application of RF energy with capacitively coupled electrodes disposed in the processing chamber. The plasma creates a highly reactive species that reacts with and etches away the unwanted deposition material present in the processing chamber.
In addition to such in situ plasma cleaning procedures and occurring far less frequently, a second cleaning procedure involves opening the processing chamber and physically wiping the entire reactorxe2x80x94including the processing chamber walls, exhaust and other areas having accumulated residuexe2x80x94with a special cloth and cleaning liquids. This cleaning procedure is commonly referred to as a wet clean, due to the liquids employed. Failure to periodically wet clean a CVD apparatus results in accumulation of impurities that can migrate onto the wafer and cause device damage. Thus, properly cleaning a CVD apparatus is important for the smooth operation of substrate processing, improved device yield and better product performance. However, the cleaning procedures reduce the availability of a system for manufacture due to the down-time required to complete the procedures.
As an alternative to in situ plasma cleaning, a remote plasma cleaning procedure may be employed. To that end, a processing chamber is connected to a remote microwave plasma system. The remote microwave plasma cleaning procedure reduces the time required to clean the processing chamber. The high breakdown efficiency associated with a microwave plasma provides a higher etch rate (on the order of about 2 xcexcm/min), compared to the etch rate of a capacitive RF plasma.
To further increase the etch rate of unwanted deposition materials, improved reactive plasma generators have been developed which provide an increased flow of reactive radicals into a processing chamber. One such reactive plasma generator is sold under the trademark ASTRON by Applied Science and Technology, Inc. of 35 Cabot Road, Woburn, Mass. 01801-1053. A description of the Astron is located at the following Internet address http://www.astex.com/astron.htm. The Astron is a self-contained atomic fluorine generator which uses a low-field toroidal plasma to dissociate a gas flow introduced into the plasma.
What is needed, however, is a reactive plasma cleaning procedure which further reduces the time required to clean a processing chamber, as compared to the prior art.
The present invention provides a method and apparatus that reduces the time required to clean a processing chamber employing a reactive plasma cleaning technique. Some embodiments of invention do so by employing an Astron fluorine source generator which is in fluid communication with a processing chamber and a supply of fluorine source gas and a supply of inert source gas. In one embodiment, a plasma is formed in the Astron from a flow of substantially pure inert source gas. After formation of the plasma, a flow of a fluorine source gas is introduced therein such that the fluorine source flow accelerates at a rate no greater than 1.67 standard cubic centimeters per second2 (scc/s2). In this fashion, the plasma contains a plurality of radicals and dissociated inert-source gas atoms, defining a cleaning mixture. Forming the plasma in the absence of a fluorine source gas flow overcomes a previously unrecognized problem. Specifically, it was found that flowing a great amount of fluorine source gas into the ASTRON reactive plasma generator would quench the plasma. As a result, the inert-source gas and fluorine source are flowed into the Astron to ensure that the ratio of the former to the latter is greater than 1:1 Specifically, it is believed that the sudden dissociation of the fluorine source gas atoms causes a pressure spike. By slowly accelerating a flow of fluorine source gas into the plasma while ensuring that the aforementioned ratio is satisfied, this problem is overcome.
Furthermore, a maximum etch rate is achieved by ensuring that the ratio of inert source gas to fluorine source gas is greater than 1:1. To that end, flow of the fluorine source introduced into the plasma accelerates until reaching a steady rate which is typically not less than 8.33 scc/s, and the inert source gas is flowed into the Astron reactive plasma generator at a first rate, typically not less than 13.33 scc/s. Preferably, however, the first rate and the steady rate are established so that the ratio of inert-source gas to fluorine source in the cleaning mixture is approximately 3:2. The cleaning mixture is then flowed from the Astron fluorine source generator to the processing chamber where it reacts with undesired contamination present therein. Typically, the fluorine source is selected from a group consisting of NF3, dilute F2, CF4, C2F6, C3F8, SF6, and ClF3, and the inert-source gas is argon. It is preferred, however, that the fluorine source is NF3.
In another embodiment of the present invention, the acceleration of the fluorine-source gas into the Astron is not critically controlled. Rather, the plasma is formed in the absence of a fluorine-source gas, by flowing the inert source gas into the Astron at a flow rate of approximately 16.67 scc/s. After formation of the plasma, the fluorine source gas is flowed into the Astron at a rate of approximately 13.33 scc/s. After allowing stabilization of the plasma for approximately two to five seconds, the flow rate of both the fluorine source gas and the inert source gas are rapidly increased to 36.67 scc/s and 25.00 scc/s, always ensuring that the ratio of inert-source gas to fluorine source gas is greater than 1:1.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures.